Does traction circle apply to dirt track tires

One thing I haven’t touched on in some of my tire articles is the old tire circle theory. It is in every major race car handling book I have on my shelves; I have no idea where it first originated. In a very quick, one sentence analysis, it says this: To operate a tire at the very limit of its traction ability, there will be a tradeoff between the amount of acceleration and the steering angle or the amount of braking and the steering angle. To use 100% of a tires traction for acceleration you won’t be able to turn the wheels, or you will break traction.

Traction circle

I think the point of the traction circle is to illustrate that there will always be the trade off in traction between side load and either acceleration or braking traction. It all boils down to tire stretch. To get the highest amount of either acceleration or braking traction the tire will stretch have to stretch only in the fore or aft direction. To get the maximum amount of side bite, the tire must only stretch in the lateral direction. In the real race world we hardly ever only load the tire in only one direction. You can experience an example of this when braking on turn entry. If the wheel is turned gradually at only a couple of degrees, brakes can be applied without spinning the car out. If the wheels are turned to far the car looses side bite and gets loose. It is because the tire is trying to be stretched in two different directions, too much. If you take your foot off the brake the tire will only be stretched in one direction, and side bite will come back. Or in the same situation, if you were to hold the brake at the same amount and straighten out the steering wheel, you will hit a point where the side bite comes back into the tire.

The same hold true on corner exit. If you are throttling up as the wheels are still turned, you will hit a sweet spot right at the edge of spinning the car out. As you start to unwrap the steering wheel to go down the straight, more throttle can be applied because there is more traction available at the tire. The measure of a skilled driver is to walk that line and keep the tire at its maximum amount of traction all the time.

Slip angle & slip ratio

The fact of the matter is that a driver that can utilize the maximum amount of traction in each tire more than the rest will usually be the driver everyone is chasing. I heard of one analogy I really liked that described this best. Think of a tire like one of those pink erasers we used to use as kids in school. If you hold it in your fingers and push it straight down on to a table, you will be able to bend the eraser only so far until it reaches it’s limit of flexibility then it will lose traction and start to slide on the table. A tire is the same way. It can only stretch so far until it reaches its limit and looses traction. The measure of a driver is to keep the car walking that line of the tires traction limit.

Til next time,

Kevin